In many applications, such as in automotive applications, so-called frequency modulated continuous-wave (FMCW) radar systems are used to detect objects (referred to as radar targets) in front of the radar device and measure the distance to the target(s) as well as the velocity of the target(s).
Usually a digital FMCW radar transceiver system includes a digital controller (often referred to as (frequency) ramp generator or sequencer) for generating frequency ramp signals, e.g. in the form of a sawtooth waveform. The digital output signal of the ramp generator is used to control (i.e. modulate) the frequency of an RF oscillator, which is included in the RF frontend of the radar transceiver. The RF oscillator may be implemented, for example, as a phase locked loop (PLL) with a multi-modulus frequency divider using a sigma delta-modulator to implement a non-integer frequency division ratio (fractional-N frequency divider). Alternatively, a digital-to-analog converter (DAC) may be used to digitally tune the frequency of a voltage controlled oscillator (VCO). Generally, the ramp generator controls the frequency of an RF oscillator to generate the frequency modulated (FM) radar signal used for detecting position and/or velocity of radar targets in front of the radar transceiver.
Modern applications such as, for example, advanced driver assistance systems (ADAS) require sophisticated, programmable schemes of subsequent frequency ramps, including, for example, sawtooth ramps with adjustable start and stop frequency, different frequency change rates, short chirps, repeated burst and pause sequences. In addition to the frequency modulation, the ramp generator may provide ramp-synchronous control and status signals, which are used by other digital or analog circuitry of the radar transceiver. For example, RF power amplifiers may be switched on and off, analog-to-digital converters (ADCs) may be enabled and disabled and other parameters of the RF-frontend may be adjusted.
A frequency ramp (and thus a single chirp) is usually defined by a fixed number of parameters such as, for example, start frequency, chirp duration, steepness, and duration of a subsequent pause (ramp parameters). Additionally, configuration parameters may be associated with the ramp parameters of an individual ramp. As the ramp and configuration parameters have to be loaded into the ramp generator in real-time, the time required to load these parameters is a lower limit of the chirp duration. Furthermore, in applications, in which a plurality of different frequency ramps is needed, memory requirements for storing ramp and configuration parameters are an issue, particularly in a single-chip implementation of the radar transceiver. Thus, there is a need for frequency ramp generators, which allows a more efficient generation of frequency ramps to be used for generating chirp signals.